The production of cellulose by Acetobacter has been the subject of intense study since at least the 1930's. In 1947, it was shown that in the presence of glucose and oxygen, non-proliferating cells of Acetobacter synthesize cellulose. Hestrin, S., Aschner, M. and Mager, J., Nature, 159:64 (1947). Since the observations of Hestrin et al., Acetobacter has been grown with the production of cellulose under a variety of conditions. For example, when grown with reciprocal shaking at about 90-100 cycles per minute, cells have been incorporated into a large gel mass. When grown under conditions in which the culture medium is agitated with swirling motion for four hours, stellate gel bodies form which are comprised of cellulose and cells. When grown as standing-cultures, a pellicle forms at the air/medium interface. The pellicle forms as a pad generally having the same surface shape and area as the liquid surface of the vessel containing the culture. Hestrin and Schramm, Biochem. Journal, 58:345-352 (1954). Hestrin and Schramm observed rapid cellulose production by freeze-dried preparations of Acetobacter containing less than 10% viable cells. These experiments, however, only measured cellulose production in shaking conditions by such freeze dried preparations over a relatively short period of three to four hours, and were run under citrate buffering conditions to control significant pH changes caused by gluconic acid produced by Acetobacter in the presence of glucose.
Polysaccharide biosynthesis by Acetobacter has been studied by several groups using non-growing cultures. In some of these studies, Acetobacter strain 1499 was grown, the cells were freed from the cellulose pellicle, resuspended in 0.01M Tris-EDTA, frozen, and then thawed as described in Hestrin and Schramm, (1954). These treated cells were used for biochemical studies under conditions that did not sustain growth of the cells, but which did preserve enzymatic activity permitting the cellulose to be synthesized by the prepared cells.
Progress in determining conditions for culturing Acetobacter for cellulose production, however, has not been the subject of wide reporting. Thus, the conditions used for culturing Acetobacter as described in U.K. patent application 2,131,701A, by Ring et al., which claims priority of U.S. patent application Ser. No. 450,324, filed Dec. 16, 1982 (now issued U.S. Pat. No. 4,588,400), are those described in Hestrin and Schramm (1954); i.e., an initial pH of about 6, temperatures in a range from 15.degree. C. to 35.degree. C. and preferably 20.degree. C. to 28.degree. C.
According to De Ley et al., "Acetobacteriacea" pp. 267-278 in Bergeys Manual of Systematic Bacteriology, Kreig and Holt, eds., 1st ed., William & Wilkins, Baltimore and London, 1984, the best carbon sources for growth in descending order are ethanol, glycerol and lactic acid. Acid is formed from n-propanol, n-butanol and D-glucose. The carbon sources described in U.K. application 2,131,701A include fructose, mannitol, sorbitol and glucose, all of which give rapid cellulose production, and glycerol, galactose, lactose, maltose and sucrose, all of which give slower growth. No growth was observed using sorbose, mannose, cellobiose, erythritol, ethanol, and acetic acid.
In U.K. patent application 2,131,701A it is desired to produce a coherent gel-like material for use as a wound dressing, after processing to remove the culture medium. To obtain this mat-like form, the culturing material is kept motionless during cell growth and cellulose production for a period ranging from a few hours to days or weeks.
Although the formation of a coherent mat or pellicle in motionless or standing culture conditions is the culture mode described in the U.K. patent application 2,131,701A, this patent further explains that intermittent agitation of the culture medium containing cellulose-synthesizing Acetobacter can control the length of the cellulose fibril produced by the microorganism. Intermittent agitation produces fibrils of finite length which is determined by the linear extension rate of the fibril by the microorganism and the period between agitative shearing of the fibril from the surface of the bacterium. Nothing, however, is disclosed about the effects of continuous agitation on the cellulose product.
The production of cellulose from Acetobacter in continuously agitated cultures is beset with numerous problems, the most difficult of which has heretofore been culture instability. This instability is demonstrated by loss of the ability to make cellulose and the gradual overgrowth of cellulose producing cells by non-producing types. Strain instability may be the result of the appearance of spontaneous mutants or variants of the microorganism that are cellulose non-producers. This appearance of non-producers apparently occurs with a frequency high enough to shift the population balance of a culture from cellulose-producing to cellulose non-producing types during growth in agitated culture. The loss of cellulose production in shaking cultures may also be merely the result of physiological factors rather than mutation to non-cellulose producing types due to genetic changes. Leisinger et al., Ueber cellulosefrie Mutanten von Acetobacter xylinum, Arch. Mikrobiol, 54:21-36 (1966). Although the cause is not known, the sustained production of bacterial cellulose in agitated culture medium has not heretofore been reported.
Cellulose negative (Cel.sup.-) strains of Acetobacter have been made by chemical mutagenesis with ethyl methane sulfonate (EMS), nitrous acid and N'-nitro-N-nitroso-guanidine (NG). When grown in static cultures, all of the EMS and nitrous acid-, and 90% of the NG-mutated strains reverted to cellulose producing types. Valla et al., Cellulose-Negative Mutants of Acetobacter xylinum, J. Gen. Microbiol., 128(7):1401-1408 (1982). Growth of mixed cultures of cellulose producing and non-producing strains in static cultures strongly favored cellulose producing strains in static cultures, whereas growth of such mixed cultures in shake flasks favored non-producing strains. Valla et al. (1982). This result lends support to the hypothesis that the cellulose mat or pellicle produced by this microorganism enables Acetobacter cells to reach the surface of static liquid medium where the supply of oxygen is abundant. Under shaking conditions where oxygen dissolution rate and low oxygen solubility limits growth, cellulose negative strains are favored because of selective aggregation of cellulose producing cells and resulting mass transfer limitation with respect to oxygen. It will thus be readily apparent that the identification and isolation of Acetobacter strains that are stable cellulose producers in agitated culture medium is of critical importance to large scale production of cellulose from Acetobacter in cultures which are concentrated enough to require agitation for sufficient oxygen supply to the medium.
Acetobacter is characteristically a gram-negative, rod shaped bacterium 0.6-0.8 .mu.m by 1.0-4 .mu.m. It is strictly aerobic; metabolism is respiratory, never fermentative. It is further distinguished by the ability to produce multiple poly .beta.-1,4-glucan chains, chemically identical to cellulose. Multiple cellulose chains or microfibrils are synthesized at the bacterial surface at sites external to the cell membrane. These microfibrils have cross sectional dimensions of about 1.6 nm.times.5.8 nm. In static or standing culture conditions the microfibrils at the bacterial surface combine to form a fibril having cross sectional dimensions of about 3.2 nm.times.133 nm.
The cellulose fibrils produced by these microorganisms, although chemically resembling, in many aspects, cellulose produced from wood pulp, are different in a number of respects. Chiefly among the differences is the cross-sectional width of these fibrils. The cellulose fibrils produced by Acetobacter are usually two orders of magnitude narrower than the cellulose fibers typically produced by pulping birch or pine wood. The small cross sectional size of these Acetobacter-produced fibrils, together with the concomitantly greater surface area than conventional wood-pulp cellulose and the inherent hydrophilicity of cellulose, leads to a cellulose product having unusually great capacity for absorbing aqueous solutions.
This capacity for high absorbency has been demonstrated to be useful in the manufacture of dressings which may be used in the treatment of burns or as surgical dressings to prevent exposed organs from surface drying during extended surgical procedures. Such uses and a variety of medicament impregnated pads made by treatment of Acetobacter-produced intact pellicles are disclosed in U.K. 2,131,701A. The pellicles of this U.K. application are produced by growing Acetobacter in a culture medium tray which remains motionless. Because Acetobacter is an obligate aerobe, i.e., it cannot grow in the absence of oxygen, production of cellulose by Acetobacter occurs at the air-liquid medium interface. Each bacterium continuously produces one fibril at the air-liquid interface. As new cellulose is formed at the surface, existing cellulose is forced downward into the growth medium. As a result, cellulose pellicles produced in static culture conditions consist of layers of cellulose fibers. Significantly, the volume of cellulose so produced is restricted by the interface between air and culture medium. The tendency of known Acetobacter strains to become cellulose non-producers when cultured under agitated conditions at increased dissolved oxygen concentration, severely limits the amount of cellulose that can be made economically. Consequently, high cellulose productivity per unit volume of vessel in extended agitated fermentations has not been previously reported.
Another problem associated with cellulose production by Acetobacter in batch culture, whether agitated or motionless, is the ability of Acetobacter to convert glucose to gluconic acid and ketogluconic acids. The pH drop associated with such acid production by the organism also limits the amount of cellulose made, particularly in batch cultures. Moreover, the production of gluconic and keto-gluconic acids removes glucose from the medium at the expense of cellulose production.
Celluloses are encountered in various crystalline forms or "polymorphs." Cellulose have varying degrees of crystallinity depending on the source of the cellulose and method of treatment. Two common crystalline forms of cellulose are "cellulose I" and "cellulose II" which are distinguishable by X-ray, Raman spectroscopy and infrared analysis as well as by Nuclear Magnetic Resonance (NMR). Cellulose I is the lattice structure for native cellulose, and cellulose II is the lattice structure for mercerized or regenerated cellulose. Structural differences between cellulose I and II contribute to differences in reactivity and many physical properties of various celluloses.
In addition to cellulose I and II, celluloses typically have some amorphous regions which are present to some extent in all native, regenerated and mercerized celluloses and which complicate structural analysis.
C-13 solid-state NMR has revealed the presence of two distinct forms of cellulose I called I-alpha (I.sub.60) and I-beta (I.sub..beta.). These forms occur in plant-derived celluloses as well as bacterial and algal celluloses. The I.sub..beta. form dominates in plant-derived celluloses whereas the I.sub..alpha. form dominates in algal and bacterial celluloses (VanderHart and Atalla, Science 223: 283-284 (1984), and VanderHart and Atalla, Macromolecules 17: 1465-1472 (1984)). These forms cannot be distinguished by X-ray diffraction but are clearly distinguishable by solid state C-13 NMR and Raman spectroscopy.